A CMOS image sensor can use a column analog-to-digital converter (column ADC) having multiple single-slope ADCs, each being arranged in a column, as a circuit for converting an analog signal, which is output from a pixel, into a digital signal. In the column ADC circuit, not only analog Correlated Double Sampling (CDS) but also digital CDS may be performed for the purpose of noise reduction. Furthermore, in order to boost the speed of the single-slope ADC, a two-step single-slope type ADC has been being used in some cases.
In the two-step single-slope column ADC circuit, a digital CDS operation cannot be performed. This degrades the image quality. It is specifically required to suppress the random noise occurring when a signal level is small.